I have a 97 F450 dump. I just picked it up from the muffler man. He took the cat off and ran a straight pipe from the downpipe back. Now when my rpms go up and stay at a constant level, I have this weird air whistle noise. It kinda sounds like an air compressor blowing off air. It never did this until the cat came off. Any ideas what this noise is and what is causing it. It will do it when truck is in park and I rev it up and keep the rpms at a constant level. It will also do it going down the road when rpms are at a constant level. Any ideas?